1. Field of the Invention
The present invention relates to a biodegradable linear random aliphatic/aromatic copolyester, in particular, to a biodegradable aliphatic/aromatic copolyester of high molecular weight and narrow molecular weight distribution, and to a process of making and use of the same.
2. Description of the Related Art
At present, aromatic polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polypropylene terephthalate (PPT) as engineering materials have been widely applied in various fields. These polymers can be made into various materials such as fibers, beverage bottles and films. However, these polymers are not substantially biodegradable, and are very resistant to bacterial or fungal attack. Until now, no bacteria or enzyme has been observed to be able to erode pure aromatic polyesters (PET, PBT). These polyesters are also difficult to degrade in water. The service life of PET can be up to 16-48 years, and PET fiber can exist in a human/animal body for 30 years. Thus, the “white pollution” that results from these polymers is an impending disaster facing humanity, and there is an urgent need to develop biodegradable materials. Aliphatic polyesters are attracting more and more attention owing to their excellent biocompatibility and biodegradability, their non-toxicity, their degradation products, etc. They have been applied to biomedical materials (skeleton fixing and supporting materials, drug controlled-release materials, nerve guide and artificial blood vessel, suture line, etc.) and environment-friendly materials (trash bag, shopping bag, food packaging, dishware, agricultural mulch film, domestic bottles and jars, fishing gears, etc.). However, compared with aromatic polyesters, the use of aliphatic polyesters has been limited due to defects such as low melting point, weak mechanical property and their high price. Therefore, the development of aliphatic-aromatic copolyesters, which combine the excellent use and processing properties of aromatic polyesters with the excellent biodegradability of aliphatic polyesters, is an important area of research regarding degradable materials.
In recent years, aliphatic/aromatic copolyesters have been commercialized abroad, mainly including Ecoflex from BASF AG, Germany; Eastar Bio from Eastman Co., USA; Bionelle from Showa Co., USA (whose resins are provided by Showa Highpolymer Co., Japan, and SK Chemicals Co., Korea), Sky Green BDP from SK Chemicals; Biomax from Dupont Co., etc.
Since these copolyesters are prepared via polycondensation, in which distilled fractions come out, it is difficult to prepare a high-molecular weight product, and in general only a product having a molecular weight of tens of thousands can be prepared. Owing to the low molecular weight, the mechanical properties of the product do not meet current standards for non-degradable materials that are widely used at present. Thus, it has become more important to prepare a high-molecular weight copolyester product having sufficient mechanical and processing properties. In U.S. Pat. Nos. 5,817,721, 5,889,135, 6,018,004, 6,046,248 and 6,114,042, BASE AG set forth that the addition of an anhydride, ether or isocyanate having multiple (at least three) functional groups reactive with polyester as a chain extender can increase the weight-average molecular weight (maybe up to hundreds of thousands) of the obtained copolyester, but the number-average molecular weight is not increased to such an extent as the weight-average molecular weight, and the molecular weight distribution of the product becomes substantially broader (3.5-8). The obtained product has a long-chain branch structure, which inhibits further processing. Moreover, the added chain extender is directly linked to the long chain of the polymer, which makes the polymer's structure complicated and impure, and the material is also hard to remove.
The preparation of polyesters prepared by polycondensation, such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and polypropylene terephthalate (PPT), involves two steps comprising: esterification or interesterification of terephthalic acid or ester-forming derivatives thereof with aliphatic diol, wherein the esterification is carried out at high pressure, and the interesterification is carried out at atmospheric pressure; and vacuum polycondensation of the esterified or interesterified product. The two steps are generally carried out separately, and two polymerization autoclaves are needed therefor.
The catalyst systems used in the preparation of polyesters by polycondensation are diversified, involving almost all subgroup A and B elements in the Periodic Table of the Elements except for halogens and the inert elements. Those having the best catalytic effect may be generalized into the following three types: Ti-series, Sb-series and Ge-series. Ti-series elements as catalysts have high catalytic activity, but the polyester product obtained has poor stability and yellow color. When Sb-series elements are used as catalysts, the polymerization reaction goes steadily. At present, 80% of polyester products are prepared using Sb-series catalyst at home and abroad. However, since Sb elements are not environmentally friendly owing to their toxicity, the service field of polyester products prepared thereby is limited. Ge-series catalysts are one type of polyester catalysts having the best overall effects, but they cannot be widely used due to their high price.
In U.S. Pat. No. 5,817,721, BASF AG discloses a biodegradable polyester, which is obtained by mixing batch-wise aromatic dicarboxylic acid or ester-forming derivatives, aliphatic diol, and aliphatic dicarboxylic acid or ester-forming derivatives, and allowing the mixture to react in the presence of Sn compounds, Ti compounds and the like as esterification, interesterification and polycondensation catalysts.
As disclosed in U.S. Pat. No. 6,018,004, U.S. Pat. No. 6,046,248 and U.S. Pat. No. 6,114,042, the preparation processes of polyesters in BASF AG are mostly carried out by two-steps in two polymerization autoclaves. First, adipic acid is esterified with 1,4-butanediol (BDO) in the presence of tin dioctoate as a catalyst thereby obtaining an esterified product. Then, the esterified product as above obtained, dimethyl terephthalate (DMT), BDO, and tetrabutyl orthotitanate (TBOT) are simultaneously added to another reactor, and after finishing the interesterification between DMT and BDO, the system undergoes polycondensation in vacuum. The molecular weight of the polyester product obtained is not high, the number-average molecular weight (Mn) being about 1×104, and the weight-average molecular weight being about 3×104. If an anhydride, ether or isocyanate having multiple (at least three) functional groups reactive with polyester is added as a chain extender to the aforesaid second step, the weight-average molecular weight of the obtained copolyester can be obviously increased, but the number-average molecular weight is not increased to the same extent as the weight-average molecular weight, and the molecular weight distribution of the product becomes broader (3.5-8).
Currently, the preparation of polyester by polycondensation generally includes three reaction stages, i.e., esterification, interesterification and polycondensation. In general, monomers and esterification or interesterification catalysts are simultaneously added to a polymerization autoclave; after finishing esterification or interesterification (which is determined upon the distilled fractions), the esterified or interesterified product and polycondensation catalyst are simultaneously added to another polymerization autoclave for polycondensation in vacuum.
The preparation of polyester by polycondensation is a relatively mature process. Currently, the compounds of Ti, Zn, etc., and the compounds of heavy metals such as Pb, Sn, Sb and Cd are generally used as catalysts at the stages of esterification, interesterification and polycondensation. The former compounds produce serious side reactions, and the polyester product obtained has poor stability and yellow color; and the latter compounds have some toxicity, thus the use of the polyester products obtained is limited. Although germanium oxide catalyst systems have relatively better effects, it is difficult to widely use them owing to their high price. Thus, it is highly desirable in the polyester industry to develop a novel catalyst system having good effects and low price.
In recent years, the use of rare earth (RE) compounds as catalysts has attracted the interest of scholars at home and abroad. There are many relevant techniques with respect to the preparation of polyesters using La-series metal catalysts, e.g., CN1,112,573A, EP0,626,425 and CN1,446,837A. In patent application No. CN1,112,573A, Enichemu Co., Italy, discloses the use of La-series metal compounds, their metal salts, metal composite salts or salt-containing complexes as catalysts, thereby obtaining a thermoplastic aromatic polyester having high resistance to degradation in the molten state. In European patent application EP0,626,425, La-series metal composite salts are used as catalysts for producing a thermoplastic aryl polycarbonate/aryl polyester component, with the thermoplastic component obtained having improved mechanical, thermal and electrical performance and relatively high stability. In Chinese patent application CN1,446,837A, a La-series metal catalyst for preparing polyester is disclosed, said catalyst including components R1 and R2, wherein R1 is La-series metal halide and/or a La-series metal complex, and R2 is La-series metal hydroxide, and the mixture of both can enable the interesterification to be carried out rapidly and stably.
China is enriched with RE elements, accounting for about 80% of world reserves. Although it is well known that the use of RE compounds as polyester catalysts can increase the rate of polymerization, the actual process and the selection of catalysts need to be further studied. In the technique of polyester preparation, there still exist many shortcomings such as complicated operation processes and relatively broad molecular weight distribution of the polymerized product. Thus, it is desirable to provide a process for preparing polyesters using novel RE catalyst systems that are capable of simplifying the present polyester preparation process and reducing the occurrence of side reactions.
At present, all polymerization catalyst systems used for preparing biodegradable aliphatic/aromatic copolyesters by polycondensation in the prior art are titanium alkoxide, alkyl tin, germanium oxide and etc., for example, tetrabutyl orthotitanate, titanium isopropoxide and etc. as disclosed in Nihon Yukagakkaishi (1999, 48(9), p 911-915), European patent EP1,106,640A2, and Germany patent DE19,923,053A1, n-butyl tin as disclosed in Korea patent KR9,709,332B1, germanium compounds as disclosed in Japanese patent JP2,004,018,674A2, and so on. As described above, the catalyst systems commonly used for polyesters still have various defects which result in product of yellow color, and produce serious side reactions.
To sum up, the existing catalysts for polyesters have many shortcomings including low polymerization rate, serious side reactions, toxicity, product of yellow color, etc. Additional shortcomings such as complicated processes and relatively broad molecular weight distribution of the polymerized product further exist in the known techniques for preparing biodegradable copolyesters.